Three different logs, the first one showing a response that does not trigger the bug.

Version:
   Qt Creator 4.9.2
   Based on Qt 5.12.4 (Clang 10.0 (Apple), 64 bit)

   Built on Jul 8 2019 00:16:52

   From revision 8181363fa9

(I tried briefly with 4.10.0-beta2 but it also barfs, at least on renameProvider)

# Working connection to LS:
-bash$ QT_LOGGING_RULES=qtc.languageclient.*=true ~/Applications/Qt\ Creator\ 4.9.2/Qt\ Creator.app/Contents/MacOS/Qt\ Creator | tee ~/qt_lsp.log
qtc.languageclient.client: initializing language server  "SPIDER Language Server"
qtc.languageclient.messages: StdIOClient send data:
qtc.languageclient.messages: Content-Length: 678

{"id":"{a03880c1-4874-44ec-b6fc-aad133506930}","jsonrpc":"2.0","method":"initialize","params":{"capabilities":{"textDocument":{"codeAction":{"codeActionLiteralSupport":{"codeActionKind":{"valueSet":["*"]}}},"completion":{"completionItem":{},"completionItemKind":{"valueSet":[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25]},"dynamicRegistration":true},"documentSymbol":{"symbolKind":{"valueSet":[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26]}},"synchronization":{"didSave":true,"dynamicRegistration":true,"willSave":true,"willSaveWaitUntil":false}},"workspace":{"workspaceFolders":true}},"processId":8959,"rootUri":null,"trace":"off"}}
qtc.languageclient.messages: StdIOClient std out:

qtc.languageclient.messages: Content-Length: 627

{"jsonrpc":"2.0","id":"{a03880c1-4874-44ec-b6fc-aad133506930}","result":{"capabilities":{"textDocumentSync":2,"hoverProvider":true,"completionProvider":{},"definitionProvider":true,"typeDefinitionProvider":false,"implementationProvider":true,"referencesProvider":true,"documentHighlightProvider":false,"documentSymbolProvider":true,"workspaceSymbolProvider":false,"codeActionProvider":false,"documentFormattingProvider":true,"documentRangeFormattingProvider":true,"renameProvider":true,"workspace":{"workspaceFolders":{"supported":true,"changeNotifications":true}},"typeHierarchyProvider":false,"callHierarchyProvider":false}}}
qtc.languageclient.parse: parse buffer pos:  0
qtc.languageclient.parse:   data:  "Content-Length: 627\r\n\r\n{\"jsonrpc\":\"2.0\",\"id\":\"{a03880c1-4874-44ec-b6fc-aad133506930}\",\"result\":{\"capabilities\":{\"textDocumentSync\":2,\"hoverProvider\":true,\"completionProvider\":{},\"definitionProvider\":true,\"typeDefinitionProvider\":false,\"implementationProvider\":true,\"referencesProvider\":true,\"documentHighlightProvider\":false,\"documentSymbolProvider\":true,\"workspaceSymbolProvider\":false,\"codeActionProvider\":false,\"documentFormattingProvider\":true,\"documentRangeFormattingProvider\":true,\"renameProvider\":true,\"workspace\":{\"workspaceFolders\":{\"supported\":true,\"changeNotifications\":true}},\"typeHierarchyProvider\":false,\"callHierarchyProvider\":false}}}"
qtc.languageclient.parse:   complete:  true
qtc.languageclient.parse:   length:  627
qtc.languageclient.parse:   content:  "{\"jsonrpc\":\"2.0\",\"id\":\"{a03880c1-4874-44ec-b6fc-aad133506930}\",\"result\":{\"capabilities\":{\"textDocumentSync\":2,\"hoverProvider\":true,\"completionProvider\":{},\"definitionProvider\":true,\"typeDefinitionProvider\":false,\"implementationProvider\":true,\"referencesProvider\":true,\"documentHighlightProvider\":false,\"documentSymbolProvider\":true,\"workspaceSymbolProvider\":false,\"codeActionProvider\":false,\"documentFormattingProvider\":true,\"documentRangeFormattingProvider\":true,\"renameProvider\":true,\"workspace\":{\"workspaceFolders\":{\"supported\":true,\"changeNotifications\":true}},\"typeHierarchyProvider\":false,\"callHierarchyProvider\":false}}}"
qtc.languageclient.client: language server  "SPIDER Language Server"  initialized
qtc.languageclient.messages: StdIOClient send data:
qtc.languageclient.messages: Content-Length: 52

{"jsonrpc":"2.0","method":"initialized","params":{}}



# Explicitly false colorProvider:
# General Messages says: "LanguageClient SPIDER Language Server: Initialize result is not valid: Expected type Object but value contained Bool->colorProvider->capabilities"
-bash$ QT_LOGGING_RULES=qtc.languageclient.*=true ~/Applications/Qt\ Creator\ 4.9.2/Qt\ Creator.app/Contents/MacOS/Qt\ Creator | tee ~/qt_lsp.log
qtc.languageclient.client: initializing language server  "SPIDER Language Server"
qtc.languageclient.messages: StdIOClient send data:
qtc.languageclient.messages: Content-Length: 678

{"id":"{b9f9639a-f59f-4d25-9a9f-7142afcd9475}","jsonrpc":"2.0","method":"initialize","params":{"capabilities":{"textDocument":{"codeAction":{"codeActionLiteralSupport":{"codeActionKind":{"valueSet":["*"]}}},"completion":{"completionItem":{},"completionItemKind":{"valueSet":[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25]},"dynamicRegistration":true},"documentSymbol":{"symbolKind":{"valueSet":[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26]}},"synchronization":{"didSave":true,"dynamicRegistration":true,"willSave":true,"willSaveWaitUntil":false}},"workspace":{"workspaceFolders":true}},"processId":8973,"rootUri":null,"trace":"off"}}
qtc.languageclient.messages: StdIOClient std out:

qtc.languageclient.messages: Content-Length: 649

{"jsonrpc":"2.0","id":"{b9f9639a-f59f-4d25-9a9f-7142afcd9475}","result":{"capabilities":{"textDocumentSync":2,"hoverProvider":true,"completionProvider":{},"definitionProvider":true,"typeDefinitionProvider":false,"implementationProvider":true,"referencesProvider":true,"documentHighlightProvider":false,"documentSymbolProvider":true,"workspaceSymbolProvider":false,"codeActionProvider":false,"documentFormattingProvider":true,"documentRangeFormattingProvider":true,"renameProvider":true,"colorProvider":false,"workspace":{"workspaceFolders":{"supported":true,"changeNotifications":true}},"typeHierarchyProvider":false,"callHierarchyProvider":false}}}
qtc.languageclient.parse: parse buffer pos:  0
qtc.languageclient.parse:   data:  "Content-Length: 649\r\n\r\n{\"jsonrpc\":\"2.0\",\"id\":\"{b9f9639a-f59f-4d25-9a9f-7142afcd9475}\",\"result\":{\"capabilities\":{\"textDocumentSync\":2,\"hoverProvider\":true,\"completionProvider\":{},\"definitionProvider\":true,\"typeDefinitionProvider\":false,\"implementationProvider\":true,\"referencesProvider\":true,\"documentHighlightProvider\":false,\"documentSymbolProvider\":true,\"workspaceSymbolProvider\":false,\"codeActionProvider\":false,\"documentFormattingProvider\":true,\"documentRangeFormattingProvider\":true,\"renameProvider\":true,\"colorProvider\":false,\"workspace\":{\"workspaceFolders\":{\"supported\":true,\"changeNotifications\":true}},\"typeHierarchyProvider\":false,\"callHierarchyProvider\":false}}}"
qtc.languageclient.parse:   complete:  true
qtc.languageclient.parse:   length:  649
qtc.languageclient.parse:   content:  "{\"jsonrpc\":\"2.0\",\"id\":\"{b9f9639a-f59f-4d25-9a9f-7142afcd9475}\",\"result\":{\"capabilities\":{\"textDocumentSync\":2,\"hoverProvider\":true,\"completionProvider\":{},\"definitionProvider\":true,\"typeDefinitionProvider\":false,\"implementationProvider\":true,\"referencesProvider\":true,\"documentHighlightProvider\":false,\"documentSymbolProvider\":true,\"workspaceSymbolProvider\":false,\"codeActionProvider\":false,\"documentFormattingProvider\":true,\"documentRangeFormattingProvider\":true,\"renameProvider\":true,\"colorProvider\":false,\"workspace\":{\"workspaceFolders\":{\"supported\":true,\"changeNotifications\":true}},\"typeHierarchyProvider\":false,\"callHierarchyProvider\":false}}}"
qtc.languageclient.client: language server  "SPIDER Language Server"  initialized
qtc.languageclient.messages: StdIOClient send data:
qtc.languageclient.messages: Content-Length: 52

{"jsonrpc":"2.0","method":"initialized","params":{}}

# Rename Prepare
# General Messages says: "LanguageClient SPIDER Language Server: Initialize result is not valid: Expected type Bool but value contained Object->renameProvider->capabilities"
-bash$ QT_LOGGING_RULES=qtc.languageclient.*=true ~/Applications/Qt\ Creator\ 4.9.2/Qt\ Creator.app/Contents/MacOS/Qt\ Creator | tee ~/qt_lsp.log
qtc.languageclient.client: initializing language server  "SPIDER Language Server"
qtc.languageclient.messages: StdIOClient send data:
qtc.languageclient.messages: Content-Length: 678

{"id":"{e77dfa38-68e8-41e9-854c-ceff3e7bcc77}","jsonrpc":"2.0","method":"initialize","params":{"capabilities":{"textDocument":{"codeAction":{"codeActionLiteralSupport":{"codeActionKind":{"valueSet":["*"]}}},"completion":{"completionItem":{},"completionItemKind":{"valueSet":[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25]},"dynamicRegistration":true},"documentSymbol":{"symbolKind":{"valueSet":[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26]}},"synchronization":{"didSave":true,"dynamicRegistration":true,"willSave":true,"willSaveWaitUntil":false}},"workspace":{"workspaceFolders":true}},"processId":9000,"rootUri":null,"trace":"off"}}
qtc.languageclient.messages: StdIOClient std out:

qtc.languageclient.messages: Content-Length: 647

{"jsonrpc":"2.0","id":"{e77dfa38-68e8-41e9-854c-ceff3e7bcc77}","result":{"capabilities":{"textDocumentSync":2,"hoverProvider":true,"completionProvider":{},"definitionProvider":true,"typeDefinitionProvider":false,"implementationProvider":true,"referencesProvider":true,"documentHighlightProvider":false,"documentSymbolProvider":true,"workspaceSymbolProvider":false,"codeActionProvider":false,"documentFormattingProvider":true,"documentRangeFormattingProvider":true,"renameProvider":{"prepareProvider":true},"workspace":{"workspaceFolders":{"supported":true,"changeNotifications":true}},"typeHierarchyProvider":false,"callHierarchyProvider":false}}}
qtc.languageclient.parse: parse buffer pos:  0
qtc.languageclient.parse:   data:  "Content-Length: 647\r\n\r\n{\"jsonrpc\":\"2.0\",\"id\":\"{e77dfa38-68e8-41e9-854c-ceff3e7bcc77}\",\"result\":{\"capabilities\":{\"textDocumentSync\":2,\"hoverProvider\":true,\"completionProvider\":{},\"definitionProvider\":true,\"typeDefinitionProvider\":false,\"implementationProvider\":true,\"referencesProvider\":true,\"documentHighlightProvider\":false,\"documentSymbolProvider\":true,\"workspaceSymbolProvider\":false,\"codeActionProvider\":false,\"documentFormattingProvider\":true,\"documentRangeFormattingProvider\":true,\"renameProvider\":{\"prepareProvider\":true},\"workspace\":{\"workspaceFolders\":{\"supported\":true,\"changeNotifications\":true}},\"typeHierarchyProvider\":false,\"callHierarchyProvider\":false}}}"
qtc.languageclient.parse:   complete:  true
qtc.languageclient.parse:   length:  647
qtc.languageclient.parse:   content:  "{\"jsonrpc\":\"2.0\",\"id\":\"{e77dfa38-68e8-41e9-854c-ceff3e7bcc77}\",\"result\":{\"capabilities\":{\"textDocumentSync\":2,\"hoverProvider\":true,\"completionProvider\":{},\"definitionProvider\":true,\"typeDefinitionProvider\":false,\"implementationProvider\":true,\"referencesProvider\":true,\"documentHighlightProvider\":false,\"documentSymbolProvider\":true,\"workspaceSymbolProvider\":false,\"codeActionProvider\":false,\"documentFormattingProvider\":true,\"documentRangeFormattingProvider\":true,\"renameProvider\":{\"prepareProvider\":true},\"workspace\":{\"workspaceFolders\":{\"supported\":true,\"changeNotifications\":true}},\"typeHierarchyProvider\":false,\"callHierarchyProvider\":false}}}"
qtc.languageclient.client: language server  "SPIDER Language Server"  initialized
qtc.languageclient.messages: StdIOClient send data:
qtc.languageclient.messages: Content-Length: 52

{"jsonrpc":"2.0","method":"initialized","params":{}}